A liquid crystal display device (LCD) conventionally comprises two glass plates attached parallel to one another, between which there is arranged a layer of liquid crystals. The two glass panels each carry electrodes on the side facing the liquid crystal layer, which electrodes may be exposed to different voltages, in order to change the optical characteristics of the liquid crystals located between the electrodes. These optical characteristics are essentially those which influence light transmitting capacity. In the case of dot matrix liquid crystal display devices, the electrodes take the form of dot-like areas (picture elements, pixels), which are connected together on the one side of the liquid crystal layer in rows and on the other side in columns. They are activated by suitable electrical circuits, which comprise a row and column driver. Such row and column drivers activate the electrodes cyclically with different voltages of different polarities. For this, a plurality of different intermediate voltages, for example, six are required. These intermediate voltages are conventionally generated by an appropriate voltage divider, the outputs of which are connected to the row and column drivers. The voltage divider typically comprises a plurality of series-connected resistors, between which the different voltage levels may in each case be picked off.
The problem to be solved by the present invention is illustrated with reference to FIG. 1, which comprises a schematic diagram of six voltage levels V1 to V6. The voltage level V1 is conventionally identical to an LCD operating voltage Vlcd, and V6 may be identical to ground. A row voltage waveform 103 and a column voltage waveform 104 typical of commercially available liquid crystal display devices are illustrated schematically. When the electrodes are activated cyclically, it is possible to distinguish between two half-periods 101, 102:
During an “even” half-period 101, the column voltage 104 is kept at the level V2 (“unselected”) or set to V6 (“selected”). For switched-on (for example black) pixels, the row driver generates the voltage V1, for switched-off (for example white) pixels the voltage V3, such that the voltage V1−V2 or V3−V2, or V1−V6 or V3−V6 is applied to the corresponding liquid crystals.
During an “odd” half-period 102, similar conditions apply, except that the voltages are mirrored in relation to an axis of symmetry 105 located at (V1−V6)/2. The voltage V6−V5 or V4−V5, or V6−V1 or V4−V1 is then applied to the corresponding liquid crystals.
For the time average for all the pixels of one column to be identical, irrespective of the number of switched-on or -off pixels in a column, the voltages arising in a half-period should be symmetrical, i.e. the following should apply:V1−V2=V2−V3; V4−V5=V5−V6.  (1)
Moreover, in an ideal case, the voltage levels should be equidistant as follows:V1−V2=V2−V3=V4−V5=V5−V6=Vd,  (2)
Wherein Vd is the constant difference voltage (equidistance). If only one of the six voltage levels deviates from the ideal value, for example as a result of production fluctuations, and violates the equidistance conditions (1) or (2), asymmetries will occur, which yield differing contributions from switched-on and switched-off pixels. This leads to undesirable image distortions, which are easily visible to the eye and reduce image quality. This type of distortion is known as “crosstalk”, because it depends on the mutual interaction of pixel contents.
Liquid crystal display devices with gray stage displays or color displays are particularly sensitive to this type of crosstalk. In these, the gray stages lie on the steep slope of the characteristic curve (VT curve) of the liquid crystal. In this case, deviations of a few millivolts from the equidistance conditions (1) or (2) are visible to the eye and perceived as disturbing.
For a complete correction of all the errors causing crosstalk by means of calibration, a circuit would be necessary which allowed an independent setting of all the voltage levels relative to a reference voltage, for example ground. However, such a circuit would be extremely expensive, would occupy a large area, and would consume a relatively large amount of electrical power. It would therefore be unsuited to practical application.
JP-A-10-062743 discloses a circuit for a liquid crystal display device, which is designed to eliminate crosstalk. The circuit is so designed that two voltage levels are always changed at the same time. This is achieved by means of two embodiments. In a first embodiment, two resistors from a plurality of series-connected resistors are changed. This requires an increased hardware expenditure and/or greater accuracy of resistor chain pick-offs. In a second embodiment, one resistor is changed from each of two parallel-connected resistor chains. This entails an increased power consumption and/or an increased space requirement.